1. Field of the Invention
This invention relates to moisture separator reheaters and more particularly to improved reheaters for moisture separator reheaters used in nuclear steam turbine power plants.
2. Description of the Prior Art
Steam derived from a fossil-fueled boiler is generally hot and dry and contains sufficient energy to operate the high-pressure turbine. Thereafter, it is generally reheated in the boiler so that sufficient useful work may be performed thereby, first in the intermediate and then in low-pressure turbine stages. Steam from a nuclear steam generator or reactor, on the other hand, is generally of relatively low temperature and is saturated. After passing through the high-pressure turbine the nuclear steam contains sufficient entrained moisture that it must be demoisturized, and preferably reheated thereby increasing its enthalpy in order that it reliably perform further useful work.
Moisture separator reheaters (MSR's) of various types are well known in the art. One example of such moisture separator reheaters is disclosed in U.S. Pat. No. 3,712,272, Carnavos et al. The moisture separator reheater disclosed in the Carnavos et al patent employs two reheater sections each of which comprises a bank or bundle of U-shaped tubes extending longitudinally within a pressure-tight shell and including a header for introducing a heating fluid (steam) to the tubes and withdrawing the fluid (condensate) from the tubes. The Carnavos header is provided with a vertical baffle disposed substantially at the middle thereof dividing the header into inlet and outlet sections, with the U-bends of the bundle disposed horizontally. Each tube has one end communicating with the inlet header and another end communicating with the outlet header. In operation, saturated heating steam is fed to the U-shaped tubes through the inlet section of the header, traverses the tubes, and exits, ideally as condensate, from the tubes through a single drain provided in the outlet section of the header.
Another example of a moisture separator reheater employing two reheat tube bundles is described in U.S. Pat. No. 3,713,278, Miller et al. In this design, the header is provided with a substantially horizontal baffle disposed substantially at the middle thereof, dividing the header into an upper inlet header and a lower outlet header. The U-bends are thus disposed in the vertical direction. A further moisture separator reheater design employing a single reheat bundle is disclosed in U.S. Pat. No. 3,593,500, Ritland et al.
Under certain operating conditions, substantial quantities of the reheating steam may condense within the most highly loaded tubes of all these moisture separator reheater designs. If all of the incoming steam to these tubes is completely condensed before the tube end, a buildup of subcooled condensate can result. Problems associated with condensate subcooling, well known to the reheater arts, include reduction in reheater performance, thermal cycling with potential tube-to-tubesheet weld failures, aggravation of tube bundle distortion problems, and in severe cases, overall system instability. It is further well known that selective restriction of certain of the tubes to match tubeside flow rate with actual heat transfer duty can reduce subcooling. Such a solution for the reduction of subcooling in steam heat exchangers is shown by U.S. Pat. No. 3,073,575--Schulenberg. Schulenberg teaches the use of an apertured plate having different sized apertures adjacent to the entrance to different tubes of a heat exchanger to adjust the quantity of steam flowing into respective tubes. In yet another arrangement U.S. Pat. No. 3,830,293 to Bell teaches the use of partitions to divide the surface of a tube plate to provide flow restrictors to provide different quantities of steam to different regions of the tube plate and restricting steam flow at different rates for different groups of tubes.
Unfortunately, "orificing", whether on an individual tube basis as taught by Schulenberg or by groups of tubes as taught by Bell, is not normally a complete answer for the problems of condensate subcooling and related instabilities in moisture separator reheaters. A principal reason for orificing not being a complete solution is that any given orificing arrangement calculated and implemented to distribute the steam flow in the respective tubes so as to satisfy the theoretical heat transfer demand for one given operating condition, normally approximately full loading of the associated turbine, is not ideal for all operating conditions. Orificing which is ideal for one set of conditions may not be suitable for a different set of conditions, e.g., as turbine loading is changed from one power level to another. Thus, for example, orificing is designed to provide a greater flow rate in the most heavily loaded tubes at full load operation in order to preclude condensate subcooling therein by ensuring a flow rate of sufficient magnitude higher than in the other tubes to ensure avoidance of excess condensate.
Tube bundle scavenging flow, well known in the art, is typically "dumped" to a lower point in the system, as described in U.S. Pat. No. 3,724,212, Bell. To substantially eliminate condensate subcooling with orificed bundles, significant quantities of scavenging steam must be drawn through the tube bundles at off-design conditions. This is largely due to reduced loading of the lower tubes with respect to the remaining tubes in the bundle, at lower loadings, rendering the high rate of scavenging flow therein superfluous. Although the quantity of scavenging steam required is less than that for bundles in which orificing is not utilized, a substantial thermodynamic loss results from the dumping of this scavenging steam to lower points in the system.
Alternative techniques to orificing are also known in the art. The use of additional baffling in reheater headers to effect a "four-pass" arrangement is disclosed in U.S. Pat. No. 3,996,897--Herzog, and in U.S. Pat. No. 3,759,319, Ritland. By means of the additional baffling, all of the inlet tubeside flow is restricted to a fraction of the tubes. Following the first two tubeside passes and drainage of condensate formed therein, the remaining steam enters the remaining portion of the bundle which serves as the third and fourth passes. By this technique, the scavenging steam in the first two passes is substantial, whereas the scavenging steam dumped from the fourth pass to a lower point in the system can be kept at a relatively low rate and still substantially eliminate condensate subcooling across the turbine load range.
In the copending application of Reed et al, Ser. No. 890,674, filed Mar. 27, 1978 and commonly assigned, a different approach is taken in substantially eliminating condensate subcooling. With the use of a high .DELTA.P thermocompressor, high rates of scavenging steam are recirculated in lower pressure reheater tube bundles. With the scavenging steam continually recirculated, the substantial thermodynamic loss associated with the dumping of scavenging steam to lower points in the system is minimized.
As is evident from the four-pass and high .DELTA.P thermocompressor arrangements cited above, high rates of scavenging steam are in many cases beneficial. In some applications, however, it is desirable to minimize the scavenging flow rate so that tubeside velocities and pressure loss are reduced accordingly. In the four-pass arrangement, with all of the incoming steam to the tube bundle entering a fraction of the bundle, the resultant high scavenging flow rate in the second pass implies high tubeside velocities and frictional pressure loss. With high scavenging flow rates via the high .DELTA.P thermocompressor, although recirculated, high tubeside velocities and frictional pressure loss are again characteristic. Therefore, while the arrangements of four-pass and scavenging flow recirculation via high .DELTA.P thermocompressors succeed in substantially eliminating condensate subcooling, potential erosion problems due to high tubeside velocities and performance degradation due to the loss in heating steam temperature associated with the high frictional pressure loss could in some instances become undesirable side effects.
In yet another technique disclosed in U.S. Pat. Nos. 3,731,734 and 3,802,496, assigned to Ris et al, adjustable selective orificing is used in steam condensers. Here the fixed resistances of orificing are dealt with by the utilization of adjustable plates mounted within the steam intake header compartment. This solution, however, necessitates a unit shutdown in order to gain access to the internal adjustable plates. During turbine load changes, for instance, the restrictions would remain fixed.
Accordingly, it is an object of the present invention to provide an improved reheater for a moisture separator reheater which simply and efficiently substantially eliminates condensate subcooling and related instabilities.
It is a further objective to substantially eliminate condensate subcooling with minimal scavenging flow rate and resultant minimal increases in tubeside velocities and pressure loss.